This invention relates to a probe for use in nuclear magnetic resonance experiments and to a method for use in such experiments.
In magnetic resonance experiments on a sample within a magnetic field, the RF field, generally known as the B1 field, is generated by an NMR probe located at or adjacent the sample. The probe can be provided by a surface coil arrangement which is generally planar and located immediately adjacent the surface of the sample for experiments on a volume of interest within the sample and spaced from the plane of the probe. Surface coils are widely used in magnetic resonance imaging (MRI) and spectroscopy (MRS) due to their high sensitivity. The sensitivity, however, decreases rapidly with increasing distance between the coil and the volume of interest due to the non-homogeneous excitation as well as signal reception. This results in variations in the intensity of MR images and adversely affects MRS experiments.
Some attempts to overcome this have used composite or adiabatic pulses applied to the probe, although these require relatively high RF power.
In the alternative, volume coils, which are no longer planar, can provide a homogeneous B1 but with a reduced signal to noise ratio (SNR) compared to surface coils. Separate transmit and receive coils are often used. In this case a volume coil or a large surface coil provides a highly homogeneous B1 field and a smaller receiving surface coil is used to obtain high SNR. However, such a configuration still suffers from inhomogeneous signal reception and the small coil must be actively de-coupled requiring cumbersome electronics that tends to introduce additional noise and reduce the SNR. In addition, high RF power is required for excitation, often limiting the pulse sequences not to exceed specific absorption rate (SAR) limits.
A dual-ring surface coil with the sensitivity and power requirements of a traditional surface coil and the B1 homogeneity of a volume coil over a targeted volume of interest has been described in a paper published by Tomanek B. Ryner L, Hoult D I, Kozlowski P, Saunders J K entitled Dual Surface Coil with High-B1 Homogeneity for Deep Organ MR Imaging in Magn Reson Imag 1997; 10:1199-1204. Further details are published in a paper published by King S B, Ryner L N, Tomanek B, Sharp J C, Smith I C P entitled MR Spectroscopy using Multi-Ring Surface Coils in Magn Reson Med 1999; 42:655-664.
This arrangement is used for 1H MR imaging and 1H spectroscopy. In this design an additional inductively coupled coaxial ring produces a B1 field that compensates for the roll-off of the B1 field produced by a larger surface coil resulting in high B1 homogeneity over a specified volume.
The combination of MR imaging with spectroscopy provides a powerful tool for clinical management of problems such as stroke, tumor monitoring and the assessment of other diseases. Especially the integrated acquisition of proton images and localized spectra is essential for the practical application of spectroscopic techniques to human and animal research and it is highly desirable to use a single RF probe in order to minimize operational problems such as changing coils from one experiment to another. Advantages of a such a probe in the form of a double-tuned coil also include more accurate localization of spectra and easier shimming for 31P spectroscopy since the shimming can be done at the 1H frequency with the same coil over the same volume. Absolute quantitation methods using proton signal are as well easier to perform when correction for substantially different B1 profile is not required.
Early publications on double resonant RF devices dealt mostly with surface coils. One example is shown in U.S. Pat. No. 4,742,304 of Schnall et al issued May 3, 1988 which discloses a frequency splitting circuit used on a simple coil which allows the coil to resonate at two separate frequencies which can be tuned to the required frequencies for the nuclei under examination. However this arrangement has the disadvantages of the simple surface coil of the inhomogeneous B1 field leading to high levels of surface effects which interfere with the analysis of the signals from the volume of interest, often requiring surgical removal of skin and overlying tissue to allow effective investigation of an underlying tissue of interest.
While the above patent thus provides an arrangement for simultaneous investigation at the two or more required different frequencies for a simple coil, this technique could not be applied to more complex coil arrangements since the formulae disclosed for calculating the values of the inductive and capacitive components for the frequency splitting circuit could not be applied to more complex coil arrangements. Experimental analysis of the values required for more complex coil constructions is simply not practical because of the complex mutual induction between the coil components.
Recent assemblies for double resonant RF devices have therefore abandoned the above patent and are instead mostly based on volume coils where coaxial inserted birdcage resonators are often employed. Such a set-up implies for volume coils usually a high power requirement and lower SNR; very often VOI for two frequencies are not the same because of different size or position of coaxial resonators.
It is one object of the present invention to provide a method for use in simultaneous nuclear magnetic resonance experiments at two different predetermined frequencies matched to the resonant frequencies of two different nuclei using a probe of the surface coil type.
The present invention provides a combination of a multi-ring surface coil in which each coil includes a multi frequency trap circuit. The present invention provides a technique for calculating the inductance and capacitance values necessary to tune the probe defined by the combined surface coils to the required frequencies, while at the same time locating the peak of maximum homogeneity at the same distance from the plane of the coil and at the same time providing acceptable Q values for effective experiments.
In the example presented herein, imaging is performed on the 1H nucleus while spectroscopy is performed on the 31P nucleus. However other frequencies can be selected for analysis of different nuclei.
Thus the B1 field for the nuclear magnetic resonance experiments is transmitted to the sample most efficiently from a resonant RF surface coil placed in proximity to the sample and connected to the RF driving apparatus. Either the same probe or a second probe can be connected to the RF receiving apparatus to receive the MR signals which are induced by the precessing magnetism of the nuclei in conventional manner.
The probe of the present invention provides the following advantages
1. Operation of the probe at two or more frequencies for both sequential and simultaneous acquisition of MR data from two or more nuclei.
2. Good or improved homogeneity of the RF field where the peak of maximum homogeneity is at the same distance from the coil for both or all of the different frequencies. This allows the following:
a) an improved spatial localization since the higher RF field homogeneity, the more precisely the voxel of interest can be chosen;
b) an improved water suppression since the higher RF field homogeneity, the better efficiency of water peak suppression procedure.
c) an improved ability to measure mean metabolite concentrations quantatively over a tissue volume since in homogeneous RF field the NMR signal directly corresponds to metabolite concentration, but for non-homogeneous RF field, its complicated non-linear spatial dependence should be taken into account.
3. Improved signal to noise ratio relative to volume coils.
4. Reduced transmitter power requirements relative to volume coils.
5. The ability to shim, that is slightly adjust, the magnetic field using the relatively stronger signal received from one nucleus (which is in the example described hereinafter the proton or 1H nucleus), while performing spectroscopy with the relatively weaker signal received from a different nucleus (which is in the example the 31P nucleus).
6. Decreased contamination from signals generated outside the volume of interest and particularly by surface material of the sample immediately adjacent the surface probe such as the skin and adjacent tissues of an animal or human sample.
In the example described hereinafter there are two rings which are circular, coaxial, straight and either parallel or coplanar. However the number of rings can be different from two. The rings do not need to be parallel or straight but in general the rings lie in or adjacent to a surface plane which defines the surface of the probe for placing immediately adjacent the surface of the sample. The number of resonant frequencies is preferably two but can be increased from that number by increasing the number of inductor-capacitor (LC trap) combinations in each of the rings.